1. Field of the Invention
The present invention relates to a display, an array substrate, and a method of driving a display.
2. Description of the Related Art
On a display such as organic electroluminescent (EL) display that controls optical characteristics of each display element by a drive current passed therethrough, image quality deterioration such as luminance unevenness occurs if magnitudes of the drive currents vary. Therefore, when an active matrix driving method is employed in this display, the pixels must be almost the same in characteristics of a drive control element for controlling the magnitude of the drive current. In this display, however, the drive control elements are normally formed on an insulator such as glass substrate, so the characteristics of them easily vary.
U.S. Pat. No. 6,373,454 describes an organic EL display employing a current mirror circuit in a pixel circuit.
This pixel includes an n-channel field-effect transistor as the drive control element, organic EL element, and capacitor. The source of the n-channel field-effect transistor is connected to a power supply line at a lower electric potential, and the capacitor is connected between the gate of the n-channel field-effect transistor and the power supply line. The anode of the organic EL element is connected to a power supply line at a higher electric potential.
The pixel circuit is driven by the method described below.
First, the drain of the n-channel field-effect transistor is connected to its gate. A current Isig at magnitude corresponding to a video signal is made to flow between the drain and source of the n-channel field-effect transistor. This operation sets the voltage between electrodes of the capacitor, equal to a gate-to-source voltage necessary for the n-channel field-effect transistor to pass the current Isig through its channel.
Then, the gate of the n-channel field-effect transistor is disconnected from its drain, and the voltage between the electrodes of the capacitor is maintained. The drain of the n-channel field-effect transistor is subsequently connected to the cathode of the organic EL element. This allows a drive current to flow through the organic EL element at magnitude almost equal to that of the current Isig. The organic EL element emits light at a luminance corresponding to the magnitude of the drive current.
As described above, when the above configuration is employed in each pixel circuit, it is possible to make the drive current flow between the drain and source of the n-channel field-effect transistor during a retention period following a write period at magnitude almost equal to that of the current Isig supplied as a video signal during the write period. Therefore, the influence of not only the threshold value Vth but also the mobility, dimensions, and the like of the n-channel field-effect transistor on the drive current can be eliminated.
However, it is difficult for the above display to make the drive current sufficiently small. If the drive current cannot be set sufficiently small in, for example, an organic EL display, each gray level within a low gray level range is displayed at a luminance higher than that to be displayed.